Fixed frequency converter switching at zero voltage

Abstract: A converter is operated with a main switch in series circuit with primary winding of the isolation transformer and an auxiliary switch for charging a reset capacitor also in circuit with the primary winding. The main switch and auxiliary switch are operated through control logic so that neither switch is ON at the same time. A predetermined dead time is provided between turning OFF the auxiliary switch and turning ON the main switch to allow the output capacitance of the main switch to discharge to zero. The power is transferred to the output during the ON and OFF time of the main switch. The power can be transferred to the output also during only the off time. In the case when the power is transferred from the primary to the secondary during both ON and OFF time, an inductor or a saturable reactor is series circuit with the secondary or a selectively controlled rectifier is used.

Description: Background of Invention 1. Field of the Invention This invention relates to DC-to-DC converters, DC-to-AC inverters and AC-to-DC converters. The major characteristic of this power conversion technique is that transfers the power to the secondary continuously, and the switching elements switch at zero voltage. 2. Description of the Prior Art There is a continuing industry demand for increasing power density, which means more power transferred in a given volume. A method for increasing the power transfer through the converter is to increase the switching frequency. Using prior art topologies such as forward or flyback, which employ "hard" switching techniques, makes high frequency operation less efficient. The switching losses associated with switching elements, which turn on when there is a voltage across them, are proportional with the switching frequency. An increase in switching frequency, leads to an increase in switching losses and an increase in level of electromagnetic interference (EMI). In order to overcome limitations in switching speeds, the prior art has devised a new family of resonant and quasi-resonant converters. In the case of quasi-resonant converters, the prior art technique consists of shaping the current or voltage to become half sinusoidal by adding additional inductors or capacitors, and to perform the switching when the current or voltage reaches zero. An example of one such circuit can be found in Vinciarelli, "Forward Converter Switching at Zero Current", U.S. Patent 4,415,959. Another family of quasi-resonant converters which switch at zero voltage is described by F.C.Lee in High Frequency Power Conversion International Proceedings (April 1987), Intertec Communications, Ventura, California. All these convertors have the disadvantage that the RMS. voltage and/or current is higher than in hard switching designs and a switching frequency that is dependent upon the input voltage and output load. This tends to limit there applications to lower power levels. Another drawback associated with prior art power conversion technique is the fact that the power is transfer to the secondary during a portion of the cycle. Common prior art topology such as forward, transfers the power to the secondary during the ON time of the main switch. In the case of flyback converter the power is transferred to the load during OFF time of the main switch. These topologies require large outpu filters in order to smooth the power transfer to the load. A further prior art can be found in EP 0 474 471 A2. The technique utilised is a conventional single ended forward convertor with a reset switch and an extra series component to allow the voltage across the main switch to resonate to zero volts. This design transfers energy to the output when the main switch is turned on. During the OFF time energy stored in the output inductor is transferred to the output. This has the disadvantage that the output inductor is not any smaller than that used in a single ended forward convertor. EP 0 416 721 A1 has a similar disadvantage, but differs in that energy is only transferred to the output when the main primary switch is turned off. This tends to result in a large output filter. What is needed is a converter which operate at constant frequency, modulating the power by varying the duty cycle, the current and voltages on the switching elements are square-wave to decrease the current and voltages stress, the transitions are done at zero voltage conditions, and the power is transferred to the output, both during the ON time and OFF time. The present invention proposes to reduce the above mentioned disadventages of the prior art by a DC-DC converter according to the features of the subject-matter of claim 1. The invention can be better visualized by turning to the following drawings: Brief Description of the Drawings Figure 1 is a schematic of a converter utilizing the power transfer methodology of the invention. Figure 2A is a schematic diagram wherein the current mirror resetting technique of prior art is illustrated for a single ended forward converter. Figure 2B is a schematic diagram wherein another prior art technique is illustrated. Figure 3 is a timing diagram of the circuit of Figure 1. Figure 4 is a schematic diagram of an embodiment of the invention similar to Figure 1 where the saturable reactor used as third switch is replaced with a synchronized switch. Figure 5 is a schematic diagram of an embodiment of the invention similar to Figure 11 where the saturable reactor used as third switch is replaced with a inductor located in series with the primary winding. Figure 6 is a schematic diagram of the zero voltage detection technique used to control the turn ON of the main switch. Figure 7 is a timing diagram of the circuit of figure 6. Figure 8 is a schematic diagram wherein the delay achieved by the third switch is accomplished by using the capacitor Cr. Figure 9 is a schematic diagram of another embodiment of the invention wherein the tapped secondary winding is replaced by a non tapped winding connected to the secondary rectifiers in flyback mode. Figure 10 is a schematic diagram of another embodiment similar to Figure 9 wherein the power transfer is achieved in a conventional flyback mode. Figure 11 is a schematic diagram of an embodiment wherein the tapped secondary winding is replaced by a non tapped winding and a bridge rectifier. Figure 12 is a schematic diagram of an embodiment wherein the output choke is split in two coupled inductors, and their leakage inductance is used to delay the current flow into the secondary during the resonant transition. Figure 13 is a schematic diagram of another embodiment wherein the center tap of the secondary is eliminated and the double rectification in secondary is accomplished by using two inductor elements. The delay of the current flow into the secondary during the resonant transitions is accomplished by using a saturable reactor in series with the flyback output diode. Figure 14 is a similar configuration as that presented in figure 13, with the exception of the saturable reactor location, which is located in series with the forward output diode. Figure 15 is a schematic diagram of another embodiment in which the main switch is replaced by two switching elements in series to reduce the voltage stress. Figure 16 is a schematic diagram in which the split output inductor is combined with two primary switching elements in series. Figure 17 is a schematic diagram in which the double rectification scheme without the center tap is associated with two primary switching elements in series. Figure 18 represent a similar embodiment like the one presented in figure 17, with the location of the saturable inductor on the forward output diode. Figure 19 represents a similar configuration such the one depicted in figure 15, with the addition of two sets of diodes in primary which will limit the maximum voltage across the primary switching elements. Figure 20 represents another preferred embodiment in which the power transferred is done through leakage inductance of the transformer in forward mode and the current through the output rectifier has a triangular waveform with both slopes soft. Figure 21 is an embodiment which represents a power transferred using interleave technique, suitable for the embodiments depicted in figure 10 and figure 20. Figure 22 is an implementation embodiment of the configuration described in figure 13 by using integrated magnetic. Figure 23 is an embodiment in which synchronized rectifiers are employed The invention and its various embodiment may be better understood by now turning to the following detailed description. Detailed Description of the Preferred Embodiments A double ended converter or a single ended flyback DC-DC or DC-AC converter is operated with a primary switch in series circuit with a primary winding of the isolation transformer and an auxiliary switch for charging a reset capacitor also in circuit with said primary winding. The main switch and auxiliary switch are operated through control logic so that neither switch is ON at the same time. A predetermined dead time is provided between turning OFF the auxiliary switch and turning ON the primary switch to allow the output capacitance of the primary switch to discharge into the input source. Current discharge into the secondary of the transformer during this time period is limited by a third switch, which may either be a saturable reactor in series circuit with the secondary, a normal inductor , a leakage inductance of the transformer or of the output inductor, discontinuous mode operation or a selectively controlled switch. The delay in the secondary can be accomplished also by using a capacitor in between the common cathodes of the diodes and the secondary tap. This is to supply energy to the output choke during transitions time, to avoid the turn ON of the forward diode before the voltage across the main switch reaches zero. The operation of the circuit of Figure 1 is illustrated in the timing diagram of Figure 3. Curve 78 is the gate voltage applied to main switch 30 illustrating On state periods 70 and Off state periods 72. Curve 80 similarly corresponds to the gate voltage applied to secondary switch 32. Off periods 74 are alternated with On state periods 76. As can be seen by comparing curves 78 and 80, a time gap T or "dead" time is provided between T1 and T2 following an ON state 76 of secondary switch 32 and before ON state 70 of main switch 30. Curve 82 represents the voltage across the main switch 30, and can be seen that at the time T1, when the auxiliary switch 32, ceases to conduct, the voltage starts to decay to zero in a resonant manner. This allows the main switch 30, to turn ON at zero voltage at the moment T2. Curve 84 represents the current flowing through the main switch 30 and the capacitance 36, which can be its own parasitic capacitance or a additional discrete capacitor. As can be seen, in between T1 and T2 the current flows through output capacitor 32, till the voltage across it reaches zero. At that moment the main switch turns ON and the current becomes positive. The curves 86 and 88 represent the currents through the rectifiers 48 and 50. As can be seen the current through 48, starts to rise prior the voltage across the main switch 30 reaches zero. This is accomplished by the saturable reactor 48. The flux through the transformer's core 58 is not symmetrical. During the OFF time of the main switch 30, the current flowing through rectifier 50 and output inductor 52, creates a DC component for the magnetic flux. As result the transformer 60 is not only used as means for power transfer, but also as a storage element. This allows a smaller output inductor 46. If the turn ratio in between the primary winding 40 and forward secondary winding 42, is Nr1 and the turn ratio in between the primary winding 40, and flyback winding 44, is Nr2, the transfer function from the input to output is described by the following equation: Vo = D (Vin/Nr1) + D (Vin/Nr2) In the case when Nr1=Nr2=Nr the transfer function becomes: Vo = 2 D (Vin/Nr) An embodiment of a simple zero voltage sensor circuit is presented in Figure 6. The circuit consist in a delay cell created with 96, R1, and 98, C1, and the driver element 100. There is a feedback resistor 94, R2, which plays a key role in operation. After the auxiliary switch 32, ceases to conduct, the voltage across main switch 30, starts to decrease, as is depicted buy curve 102 in Figure 7. The variation of the voltage in drain of the main switch 30, causes a current through drain to gate capacitance, which is directed from the gate towards the source. This current flows through 92 and 94. The portion of current flowing through 94, discharges the filter capacitor 98. As result of it the voltage at the input of 100, is delayed further from reaching the threshold at which 100 is activated. When the voltage in drain of main switch 30, reaches zero or the bottom of the resonance, the current through 94, reaches zero or changes the polarity. This will allow the voltage at the input of 100, to reach the threshold and to turn ON the main switch 30. Figure 4 presents an implementation of the circuit from Figure 1, wherein the saturable reactor is replaced by a synchronized rectifier switch 62. This implementation is advantageous in operation for high output voltages, where the output current are not very high and the volt-second for the saturable reactor 52 of Figure 1, are large. Another embodiment, is the use of a inductor in the primary side of the circuit, as is presented in Figure 5. The presence of the inductor 90, is going to ensure the current path through 36, 40, 90 towards the input source 56. This implementation is desirable for application where the currents in primary are not very high, and as result the energy stored in 90, is a small portion of output power. The inductive element 90, can be also placed in the secondary portion of the circuit in series with the secondary winding or it can replace the output saturable reactor. In figure 8 is presented another technique of creating a delay of the secondary current. The capacitor 120 is placed in between the secondary winding tap and the common cathode of the rectifiers D2 and D3. This output current delay operates as follows. At the moment when the voltage in anode of rectifier 50, decreases the device ceases to conduct. The energy for the output inductor 52, Lo, is provided by the capacitor 120. The voltage across the 120 starts to decay towards zero, but during the transition time, the voltage in anode of D2 has to be larger than the voltage across 120 for 48 to conduct. As a result 120, introduces a delay time for the rectifier 48 to turn on. This technique is also suitable for medium and high output voltages. Another embodiment is presented in figure 9. This is a derivation of the circuit presented in Figure 1, wherein the forward portion of the secondary winding is removed. This circuit operates in a boost mode only. The delay circuit 46 is also required to avoid the turn ON of 48, D2, during the transition of the voltage in drain of 30 to zero. This power processing technique is suitable for multiple outputs, where 52, Lo, can be or not a coupled inductor. Another embodiment is presented in Figure 10. This is derived from the circuit presented in Figure 9 and represents a flyback configuration, wherein the reset capacitor 38, C1, and the switching elements 34 and 32, create an effective snubber circuit, which recycles the energy contained in leakage inductance of transformer 60, and also offers condition for zero voltage turn On for all the primary switching elements. By sizing the leakage inductance of transformer 60, the output rectifier 50 will have a soft turn off conditions even for flyback continuous mode of operation. Figure 11 shows another embodiment, wherein the tapped secondary winding is replaced by a single winding, and the output rectification is performed by using a bridge rectifier 124. This configuration is suitable for high output voltage applications. In Figure 12 is presented a preferred embodiment in which the current delay in the secondary is achieved by splitting the output inductor in two coupled inductors. The forward current will be shaped in trapezoidal form, which will help not only in delaying the current in secondary, but will create low dI/dt through the output rectifiers minimizing the reverse recovery losses. In Figure 13, the center tap is eliminated by using two output inductors 144, Lo1, and 146, Lo2. The elimination of the center tap will give a better utilization of the output winding. The configuration is compatible with integrated magnetic as is suggested in Figure 22. The diodes can be replaced with synchronized rectifiers as is depicted in Figure 23. In Figure 15, 16, 17, 18, 19, the main primary switch is replaced by two switching elements in series to reduce the voltage stress on each element. The configuration in secondary are similar to those used for only one primary switching element. In Figure 20 is presented a forward type configuration with energy transfer through the leakage inductance. The current through Do will have both slopes slow rising reducing the high frequency content of the output current. This topology is a perfect candidate for interleaving technology depicted in Figure 21, in which each power train transfers the energy successively to the load. In Figure 22 is presented a implementation of the configuration depicted in Figure 13, in which the secondary is reduced to one turn and the output inductors are implemented in the outer legs which are gapped. Many alterations and modifications may be made by those having ordinary skill in the art without departing from the scope of the invention. For example, any kind of switch now known or later devised may be used, including N or P type FET power devices which may be substituted for the main and auxiliary switches, the carrier type of the main and auxiliary switches may be interchanged with each other, and the reset capacitor may be connected to the positive source or return.

Claims: A DC to DC converter of the single ended forward type with a reset switch and an output winding, consisting of a DC voltage source (56), a transformer (60) having a primary (40) and a secondary winding (42) and main switching means (30) for selectively coupling the voltage source across the primary winding (40) of the transformer (60) and further comprising auxiliary switching means (32) for selectively allowing the voltage across the primary winding (40) to reach zero, the main switching means (30) and the auxiliary switching means (32) each having an OFF period and an ON period with the auxiliary switching means (32) being non conductive prior to the ON period of the main switching means (30) by a predetermined time period sufficient to allow the voltage across the main switching means (30) to reach zero voltage with the auxiliary switching means (32) remaining nonconductive until the main switching means (30) cycles through its corresponding ON period and half its OFF period; a reset capacitor (36) coupled between the auxiliary switching means (32) DC ground of the power source (56); third switching means (62, 63) coupled in series with the secondary winding (42) of the transformer (60) for selectively remaining nonconductive as long as the voltage across the main switching means (30) decreases, said third switching means (62, 63) becoming conductive when the voltage across said main switching means (30) is basically zero; a first rectifier means (48) coupled in series with said third switching means (32) for conducting current from said third switching means (62, 63), a second rectifier means (50) forming at one end and a common connection with the first rectifying means (48) and connected at the other end to the free end of said transformer secondary (42); a control circuit (100, 101) for selectively operating said main (30), auxiliary (32) and third (62, 63) switching means to transfer energy through said DC to DC converter by modification of the duty cycle of at least the main and auxiliary switching means (33, 32), during the ON and OFF time of said main switching means (30) turns on at basically zero volts, characterized in that said transformer (60) has a tapped output winding (42, 44), that said second rectifying means (50) is connected to the free end of said tapped winding (44) and forms a common connection with the first rectifying means (48). A DC to DC converter according to claim 1, characterised in that the first rectifying means and third switching means comprises a unidirectional controlled switch means (62), selectively operated by the control means (101) so the unidirectionally controlled switching (62) means is nonconductive until the voltage across the main switching means (30) reaches zero and is then conductive during the corresponding ON time of said main switching means (30). A DC to DC converter according to claim 1, characterised by a plurality of tapped secondary windings (42,44). A DC to DC converter according to claim 1, characterised in that the third switching means (62, 63) is a saturable reactor (63). A DC to DC converter according to claim 1, characterised in that the third switching means (62, 63) is an inductor (64). A DC to DC converter according to claim 1 characterised in that the said third switching means (62,63), is replaced on the secondary side by a connection and moved to the primary side, such that it is connected is series with the primary winding (40) of the said transformer (60). A DC to DC converter according to claim 1, characterised by a load (52) and a load inductor (54), one end of the said inductor (54) being connected to the common connection from the rectifying means (48,50), and the other end being connected the said load (52), the other end of said load (52) being connected to the common connection of the transformer secondaries (42,44). A DC to DC converter according to claim 1, characterised in that the third switching means (62,63) is removed and further characterised with the addition of a load (54) and two in phase coupled inductors (128,134), which are connected at one end together and to the load (54), the other ends being connected to the cathodes of the first (48), and second(50) rectifying means, by a third rectifying means (126) with its cathode connected to the cathode of said first rectifying means (48) and its anode connected to the common connection of the transformer secondaries (42,44) and the free end of said load (54). A DC to DC converter according to claim 8, characterised with the third switching means(63) being the leakage inductance (90) between the primary (40) and secondary (42, 44) windings of the isolation transformer (60). A DC to DC converter according to claim 1, characterised by a single secondary winding (138) with a series connected third switch (63), by a first rectifying means (48) connected to the free end of said third switch (63), by a second rectifying means (50) connected to the other end of said secondary winding (138), with the other end of said second rectifying means (50) forming a common connection of cathodes or anodes with first rectifying means (48), by two supplementary inductors (144,146), and a load (54), the inductors connected in series, with the non common ends connected to the ends of said secondary winding (138), the common ends connected to said load (54), the other end of said load (54) being connected to the common end of said rectifying means (50). A DC to DC converter according to claim 10, wherein the two supplementary inductors (144,146) are implemented on the outer legs of the gapped transformer core (58). A DC to DC converter according to claim 10, characterised by the first and second rectifying means (48,50) , being controlled rectifying means (182,184). A DC to DC converter according to claim 6 or 10, characterised with a control means (100) that senses the input voltage, input current, and output voltage, said control means (100) maintaining a constant voltage across said load (54), and forcing the input current to follow a desired shape by varying the ON period of main switching means (30). A DC to DC converter according to claim 10, characterised by, first (48) and second (50) rectifying means being replaced with a full wave rectifying means (124), one AC input, being connected to the free end of secondary winding (138), by other AC input connected to the free end of third switching means (63). A DC to DC converter according to claim 14, characterised with an output inductor, one end being connected to the plus point of said rectifying means (124), by a load (54), connected between said output inductor and minus of said rectifying means (124). A DC to DC converter according to claim 1, characterised by a control means (100) that senses the voltage across said main switching means (30), and turns ON said main switching means (30) at basically zero volts. A DC to DC converter according to claim 1, characterised with a second main switching means (152), connected between said voltage source (56), and end of said primary winding (40), that is not connected to first main switching means (30), by said reset capacitor (38), connected to said reset switching means (32) and said primary winding (40), with said reset switch (32) with the anti-parallel diode(34) connected to the other end of said primary winding (40) in series with said capacitor (38).

Inventors: BELDIANU VASILE, JITARU IONEL DAN, BELDIANU, VASILE, JITARU, IONEL DAN
Applicants: MELCHER AG
Application year: 1994
Main classification: H02M3
IPC codes: H02M3